1. Field of the Invention
The present invention relates to polymer films. The present invention particularly relates to polypropylene films useful in packaging applications.
2. Background of the Art
Polypropylene and polyethylene films are commonly used as packaging materials. Applications for such materials include use as labels for plastic containers such as shampoo and soft drink bottles, and plastic bags and wrappings such as those used for candy and other food items. Labels produced from polypropylene or polyethylene films are desirable because they are clear and extremely thin films, thus providing a look that is similar to silk-screening on the surface at a much lower cost. The polypropylene or polyethylene labels impart a clear, no label appearance to the plastic bottle. Polypropylene and polyethylene films used in these applications typically have a thickness of approximately 1.1 to 3.2 mils (27.9 to 81.3 micrometers) and can become easily deformed. Often, they are used in conjunction with one or more substrate materials.
Polyolefin films can be prepared as opaque films rather than just as transparent films. In some applications, opacity is a desirable property for such packaging films. Opacity can protect materials from being degraded by light. For example, packaged foodstuff can be subject to deterioration caused by exposure to light, particularly light having a wavelength of up to about 450 nm. Even when a degree of opacity is present in the film, spoilage may occur if the film allows passage of too much light, therefore highly opaque films are the most desirable for these purposes.
Another advantage of opaque films is that they can be used as a background for transparent labels. Opaque films are not always the most desirable of surfaces for printing. It is known in the art of preparing, for example, food wrappers to print a label on a transparent film and laminate or coextrude that film with an opaque film so that the label is displayed against an opaque background.
Typically, such opaque polymeric packaging films are multi-layer films which comprise an opaque, thermoplastic polymeric core layer having one or more skin layers thereon. The skin layers contribute various desirable characteristics to the packaging film such as heat sealability, improved appearance, printability or enhanced machine handling capabilities and the like.